The present invention relates to an optical head apparatus that is applicable to an optical disc apparatus such as an optical file apparatus and the like.
This type of optical head apparatus should preferably have as small a number of parts as is possible in order to facilitate the miniaturization and assembly of the apparatus.
FIG. 1 shows a conventional type of an optical head apparatus that is applicable to an optical magnetic disc apparatus (an optical disc apparatus).
A magnet 12 records information (binary data) with respect to an optical magnetic disc 11. A laser beam 14 emitted from a semiconductor laser element 13 passes through an optical system comprising a lens 15, a beam splitter 16 and an objective lens 17 and is focussed onto a surface of the optical magnetic disc 11. The laser beam 14 focussed onto the surface of the optical magnetic disc 11 is reflected. The reflected beam 18 reflected by the surface of the optical magnetic disc 11 has status (such as polarization status, for example) in accordance with the information recorded magnetically onto the optical magnetic disc 11. The reflected laser beam 18 passes through the objective lens 17, is reflected by a beam splitter 16 and then passes an 1/2 wavelength plate 19. When the reflected laser beam 18 passes the 1/2 wavelength plate 19, the plane of polarization is rotated 45.degree. and the proportions of the P-polarized light component and the S-polarized light component become approximately 1:1. The reflected laser beam 18 that has passed 1/2 wavelength plate 19 then passes through lenses 20 and 21 to reach a polarizing beam splitter 22. The polarizing beam splitter 22 separates the irradiated reflected laser beam 18 into the two components of an S-polarized light beam 18s and a P-polarized light beam 18p.
The P-polarized light beam 18p is irradiated to a light receiving element 23. This light receiving element 23 is divided into two elements and each of the two element output detection signals of level in accordance with the amount of light received. The S-polarized light beam 18s separated by the polarizing beam splitter 22 passes through a cylindrical lens 24 and is irradiated to a light receiving element 25. The cylindrical lens 24 operates as a lens for one direction only and has not lens operation for light entering it at right angles and so an astigmatic difference is generated in a laser beam that has passed through the cylindrical lens 24 difference. The astigmatic difference refers to the difference between the focussing position in a certain direction and the focussing position in a direction perpendicular to that. In addition, the light receiving element 25 is divided into four elements and each of these elements output detection signals with levels in accordance with the amount of light received.
The detection signals from each of the elements of the light receiving element 23 are input to a differential amplifier 26 and an adder 28. A tracking error signal is output from an output terminal 20 of the differential amplifier 26. The tracking error signal expresses the amount of difference of the position for the irradiation of the laser beam from the track on the optical disc 11. The detection signals output from each of the elements of the light receiving element 25 are input to an operation circuit 32. This operation circuit 32 generates a focus error signal on the basis of the Astigmatic Method. The focus error signal expresses the out-of-focus quantity of a spot which is formed on the optical magnetic disc 11 by the laser beam. The focus error signal from the operation circuit 32 are output from a focus error terminal 31. The output signal from the adder 28 are input to the non-reversed input terminal of a differential amplifier 27 and the output signal from the operation circuit 32 is input to a reversed input terminal of the differential amplifier 27. The reflected laser beam 18 reflected by the surface of the optical magnetic disc 11 is polarized in accordance with the recorded information (into either a P-polarized light component beam or an S-polarized light component beam) and so reproduction signal in accordance with the recorded information is output from the output terminal 29 of the differential amplifier 27.
In a conventional optical head apparatus having the configuration as has been described above, the cylindrical lens 24 and the polarizing beam splitter 22 are provided separately in a case (not indicated in the figure). Accordingly, it is troublesome to both assemble them and to perform adjustments such as the alignment of the optical axes. In addition, there is also a relatively large number of parts in the optical system.